Question

1. What is the role of permease in lactose utilization?

2. A super-repressor mutant makes a repressor where one domain functions and one does not. Which one does not function?

Trp Operon Questions

1. Briefly describe the Trp operon—what are the structural genes, what are the regulatory sequences.

2. If you had a mutant strain of E. coli that made a repressor with a nonfunctional allosteric domain, what would the effect be on gene expression?

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Answer 1

Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as the lactose in the same direction into the cell.

"super" repressor, is always bound to the operator, always preventing transcription. It has two possible causes- repressor can`t bind to the inducer and the second one is repressor can bind the inducer but can't undergo the conformation change required to get the repressor to fall off of the operator. Probably the regulatory domain which binds allolactose, an allosteric effector molecule does not function.

The trp operon is expressed (turned "on") when tryptophan levels are low and repressed (turned "off") when they are high. The trp operon is regulated by the trp repressor. Trp operon contains five structural genes: trpE, trpD, trpC, trpB, and trpA, which encode enzymes for the metabolism of tryptophan. It also contains a repressive regulator gene called trpR. trpR has a promoter where RNA polymerase binds and synthesizes mRNA for a regulatory protein. The protein that is synthesized by trpR then binds to the operator which then causes the transcription to be blocked. In the trp operon, tryptophan binds to the repressor protein effectively blocking gene transcription. When tryptophan is present, the repressor dimers bind to tryptophan, causing a change in the repressor conformation, allowing the repressor to bind to the operator. This prevents RNA polymerase from binding to and transcribing the operon, so tryptophan is not produced from its precursor. When tryptophan is not present, the repressor is in its inactive conformation and cannot bind the operator region, so transcription is not inhibited by the repressor. Another mechanism is that of attenuation. The repression system targets the intracellular trp concentration whereas the attenuation responds to the concentration of charged tRNA^trp..

the allosteric domain acts as a switch that sets the DNA-binding domain in one of two modes: functional or nonfunctional. When allosteric effectors bind to the allosteric site, they cause a conformational change in the regulatory protein so that it alters the structure of the DNA-binding domain. Now when the allosteric domain is non-functional then it would not be able to alter the structure of DNA binding domain.